In real devices with rotating rotors, mass distributions which are not completely rotationally symmetrical give rise to centrifugal and bearing forces which lead to elastic deformation of the rotor and are also the cause of vibrating of the device and the production of noise. As the speed of the rotor rises, the centrifugal and bearing forces increase. However, if the speed of the rotor exceeds a critical speed referred to as the resonance speed, then the deformation of the rotor decreases again as the speed rises. Above the resonance speed, what is known as the limit speed occurs in many devices.
Once the speed of the rotor has passed through the limit speed, it is possible to discern undamped natural oscillations of the rotor that are due to external excitations such as for example to external impacts for which friction mechanisms on the rotating rotor, sliding bearing effects, etc. may be responsible. These natural oscillations build up more and more intensively over time and can lead to instability of the device or to damage thereto.
For these reasons, during operation of the device, a value which is above the resonance speed and below the limit speed is preferably selected for the speed of the rotor. In this case, efforts are made to keep the resonance speed as low as possible, both for reasons of volume during operation and for reasons of runnability, whereas the limit speed should be set as high as possible in order to ensure a dynamically stable operation of the device even at high speeds. In particular when the device is a centrifuge or a part of a centrifuge, high speeds are desirable, as these are accompanied by a high separative power of the centrifuge.
In order to avoid unstable states of the device and possible damage thereto, it is known to damp oscillations excited by external impacts. In damping, a distinction is drawn between internal and external damping. Internal damping is determined, as a result of structural design, by the clamping fits, friction, play and rigidity of all rotating components. External damping, on the other hand, is directly dependent on the mounting.
In addition to the rigidities and points of application of the mounting, the damping of the material used has, in particular, a direct influence here. The limit speed is, in turn, dependent on the external damping-to-internal damping ratio, since whereas internal damping has a co-rotating, and therefore destabilizing, effect, external damping has a stationary, and therefore stabilizing, effect. For mounting the device, preference is therefore generally given to a mounting means which has resilient properties and the tasks of which include the damping of unbalance-excited oscillations and also the avoidance of extreme bearing forces.
Usually, a mounting means of this type has rubber buffers, which are arranged rotationally symmetrically around the axis of rotation of the rotor, as mounting elements. Although rubber buffers are inexpensive, they have the drawback that, in rubber, damping properties and spring properties are dependent on one another within narrow limits and cannot be set independently of one another in any desired manner so that, in rubber buffers, the effectiveness of the damping is subject to limits imposed by the type of material. Although it is possible to get around the aforementioned restrictions using spring systems with a parallel, for example viscous, damper, spring systems of this type are comparatively expensive.
Therefore, there is a need to increase, in a device with a rotor which is oriented perpendicularly during operation, the limit speed in a cost-effective manner.